1. Field of the Invention
The present invention relates to composite electronic components defining duplexers for use in portable telephones, for example, and in particular relates to a composite electronic component including a surface acoustic wave device that has a package structure having an electromagnetic shielding effect.
2. Description of the Related Art
The size and weight of mobile and cellular telephones have recently been reduced, and electronic components used therein also have been miniaturized, reduced in height, and combined. When the combination and high-packaging density of electronic components are promoted, signal interference is likely to arise within the device. Therefore, the prevention of signal interference is an important aspect in an electronic component and a composite electronic component formed by combining a plurality of electronic components.
To prevent signal interference, various measures have been taken. FIG. 16 is a schematic sectional view illustrating a surface acoustic wave filter as an example of a conventional surface acoustic wave device. In a surface acoustic wave filter 101, a surface acoustic wave element 103 is accommodated within a package 102. The package 102 includes a package body 104 and a lid 105. The package body 104 is formed of an insulating ceramic such as alumina. The package body 104 is provided with a plurality of electrodes that are connected to the surface acoustic wave element 103, and via bonding wires 106 and 107, the surface acoustic wave element 103 and these electrodes are electrically connected together.
In the plurality of electrodes provided in the package body 104, an electrode 108 is connected to the ground electric potential, and a terminal to be connected to the ground electric potential of the surface acoustic wave element 103 is connected to the electrode 108 via a bonding wire.
The lid 105 made of a metal protects the surface acoustic wave element 103 accommodated in the inside from external electromagnetic waves. The lid 105 is electrically connected to an electrode to be connected to the ground electric potential of the surface acoustic wave element 103 and an electrode of the package body 104, such that noises from the outside of the package are prevented from entering therein. Such a structure is disclosed in Japanese Unexamined Patent Application Publication No. 12-049565, for example.
In a composite electronic component including the surface acoustic wave device described above, similar measures have also taken. For example, in Japanese Unexamined Patent Application Publication No. 9-181567, a duplexer is schematically shown in FIG. 17. In the drawing, on a planar case circuit board 121, surface acoustic wave devices 122 and 123, capacitors 124 and 125, and a coil 126 are mounted. A metallic cover member 127 is provided on the case circuit board 121 and is fixed thereto so as to surround these elements.
In addition, although the surface acoustic wave devices 122 and 123 are schematically shown, the top surface thereof is made of a conductive material to provide an electromagnetic shielding effect similar to the surface acoustic wave device 101 described above.
In the duplexer, due to the package structure of the surface acoustic wave devices 122 and 123, a surface acoustic wave element is shielded against electromagnetic noises, and moreover, noises from the outside are prevented from entering therein by the metallic cover member 127.
In a mobile or cellular telephone, the product is constructed by combining a plurality of electronic circuit blocks that perform different functions. FIG. 18 is a circuit block diagram of a mobile or cellular telephone.
As is apparent from FIG. 18, high-frequency signals from a base station are received by an antenna 131 and are converted into IF signals by a duplexer 132 via receiving components 133 to 137. In contrast, IF transmitting signals produced in the terminal are output to the antenna 131 via the duplexer 132 after passing through transmitting components 138 to 142.
In this case, in a receiving system which handles weak signals, an RF receiving block 143 for filtering and amplifying high-frequency signals and an IF receiving block 144 for filtering and amplifying IF signals are shielded against external noises by covering the respective entire blocks with conductive cover members having the ground potential.
By connecting the conductive cover member mentioned above to the ground potential, not only is the electromagnetically shielding provided as described above, but also the ground potential in a composite electronic component is reinforced.
As described above, by connecting the lid of the surface acoustic wave device package to the ground potential, external noises are prevented from entering the package. Likewise, in a composite electronic component such as a duplexer, by connecting the conductive cover member to the ground potential, external noises are prevented from entering the composite electronic component and the ground potential is reinforced.
On the other hand, in a surface acoustic wave device such as a ladder type filter having a parallel arm resonator and a series arm resonator, large changes in characteristics due to an inductance component existing between the parallel-arm resonator and the ground potential are present. In the surface acoustic wave device, the inductance component is produced in a bonding wire for achieving the connection between an electrode which is connected to the ground potential and an electrode pad on a surface acoustic wave element which is connected to the ground potential, a via hole electrode in the package, or a wiring pattern.
Furthermore, in a composite electronic component having a surface acoustic wave device mounted thereon, various inductance components are provided to allow signals to pass through, such as a via hole electrode provided on a case circuit board and a patterned conductive pattern.
Therefore, the resultant total inductances existing between the parallel arm resonator and the ground potential greatly effect the characteristics of the surface acoustic wave device and the composite electronic component using the surface acoustic wave device.
Reduction in the height of an electronic component to be mounted is necessary with the miniaturization and reduction in thickness of a mobile or cellular telephone. Therefore, due to a physical impact and stress applied to a mobile or cellular telephone and conductive floating substances, such as solder scraps and dust, the conductive member providing electromagnetic shielding may come into contact with the conductive cover member of the composite electronic component in the package of the surface acoustic wave device. Accordingly, when these members come into contact, the parasitic inductance component is changed, resulting in deterioration in characteristics of the surface acoustic wave device.
That is, in a composite electronic component having a surface acoustic wave device built therein with a ladder-type circuit structure shown in FIG. 19, for example, there are inductances L1 to L3 generated by bonding wires in the surface acoustic wave device, an inductance L4 generated by wiring or a through-hole electrode in the package of the surface acoustic wave device, and an inductance L5 generated by wiring disposed in a case of the composite electronic component. In addition, in FIG. 19, numeral P1 denotes a ground terminal of the surface acoustic wave device package. When a conductive member providing electromagnetic shielding comes in contact with a conductive cover member of the composite electronic component so as to be connected, a connecting route shown in a dotted line X is formed. Therefore, the inductance between the parallel arm resonator in the surface acoustic wave device and the ground potential is changed, resulting in deteriorating in characteristics as mentioned above.
In addition, not only in a surface acoustic wave device with a ladder-type circuit structure having a parallel arm resonator, but also in a surface acoustic wave device with another structure, deterioration in characteristics may be produced by changes in the inductance component due to a signal route in the surface acoustic wave device and a signal route in the composite electronic component.